NATURE [FEBRUARY :,, 1914 plosive eruption such ~s excavated these gigantic stituent on the above hypothesis. It comes out craters, its first effect will be to fill up the crater approximately as 1-0 x rn-'16 sec-'. The percentage of before overflowing the edges. this active constituent would appear to be about 0-7. Lateral outpourings can only occur when the cone It is also possible to estimate the time for this com• has been sufficiently rebuilt, above the level of the position to have been attained, starting from the pure surrounding country, to give enough hydrostatic force active constituent. The time appears to be about to rend this cone. r-6 x I09 years. The radiating rays around these craters cannot be The view that thorium possesses a radio-active con• lava streams, as these only flow out of the crater by stituent as determined above may, of course, be made its lowest lip. They are not due to landslips of the the basis of an independent hypothesis. loose ejecta collected on the slopes of the cone, such J. JOLY. as I described and figured in my book on the great J. R. COTTER. Vesuvian eruption of 1906, and which had until then Trinity College, Dublin, February 3. been attributed to water erosion, for the following reason. These ravines, like the depressions around A Curious Ice Formation. a half-opened umbrella, are straight radially and not I AM taking the liberty of enclosing a photograph sinuously radial as in those surrounding the great of an occurrence which, so far as I am aware, is craters of the moon. quite unique for this part of the country, and will no Were these radial rays lava streams, which origin• doubt have some interest for your readers. ally issued from a cone now truncated by a later The water was frozen during the night of December explosive eruption, then they would have been 31, 1913 (on which night at least 14° of frost were obliterated by the enormous mantle of fragmentary registered) into circular floes of ice of varying materials that would have been ejected. diameter, which, being encrusted with snow, had the These rays have more the appearance of erosion appearance of water-lilies. valleys, but this we cannot admit if physicists main• tain that there is no lunar atmosphere to speak of. Their greatest resemblance, however, is with the irregular, radial cracks formed around the splash of a missile striking a comparatively hard surface, such as is observable when bullets are fired into soap, hard clay, lead, or half-set plaster, or even steel. The more I compare the moon's surface with vol• canic vents in different parts of this world the less I see a resemblance between the two, and the more does the planetoid and meteorite projectile theory become accept:i ble. The obviously asymmetrical craters with hitvery rigidly fixed preconceived ideas. The river, I may mention, is the Usk, and the How often have I wished to be able to study care• photograph was taken at Brecon. fully the moon's surface, and no doubt astronomers D. J. PHILLIPS. have often craved for a more extensive vulcanological University College of South Wales and knowledge. H. J. JOHNSTON-LAVIS. Monmouthshire, Cardiff. Villa Lavis, Beaulieu-sur-Mer, January 26. Soil Protozoa. The End-product of Thorium.-A Suggestion. IN a letter to NATURE (No. 2266, vol. xci., 1913) one of us (C. H. M.) gave an account of a method of THE chemical composition of thorites and thorianites obtaining permanent preparations of Protozoa in the does not seem to suggest any probable end-product for state in which they were living in the soil. the contained thorium. It has occurred to us that The fixative used in this method was picric acid in the only explanation at present available is that the saturated aqueous solution, but we have since found end-product is an isotope of thorium itself. This con• this reagent to be less serviceable in the case of clay dition might be brought about by the emission of soils than the following mixture :-Saturated aqueous sufficient f3 rays. solution of mercuric chloride, 1 pt. ; methylated spirit, If this be the case, thorium, as we know it, must 1 pt. The soil should be crumbled into this fluid, and be a mixture of two isotopic elements, one of which is mixing is best accomplished by gently shaking the radio-active. There is some support in favour of this containing vessel, care being taken to avoid making suggestion to be found in the erratic position of the clay component of the soil pass into suspension. thorium on the Geiger-Nuttall curve (Phil. Mag., A delicate film containing Protozoa will appear on October, 1912). According to this curve, the value of the surface of the liquid, and this can be removed by ,\_ for thorium, as observed, is too low. Now, if there floating cover-slips over it, and stained by the usual is _a stable component present, this result will naturally methods. K. R. LEWIN. anse. C. H. MARTIN. From the position of thorium on the diagram it is Lawes Experimental Laboratory, Rothamsted, possible to estimate the value of ,\ for the active con- January 27. NO. 23IO, VOL. 92] © 1914 Nature Publishing Group